Angiogenesis is an important process in the progression of cancer. In the absence of vascular support, cancer cells will become necrotic or even apoptotic. Therefore, antiangiogenic therapy becomes a promising strategy for treating cancers. Angiogenin (Ang) is a protein predominantly secreted by cancer cells for angiogenesis. Blocking its interaction with Ang receptors on endothelial cell surface can disable its function of inducing cancer angiogenesis and thus inhibit the progression of cancers. Although some Ang inhibitors have been developed and even approved by the FDA for blocking the interactions between Ang and its receptors to treat cancer, these inhibitors lack the tumor-homing capability and may lead to serious side effects, such as bleeding and hypertension. To address this problem, we propose to develop a human-safe phage-based tumor-homing Ang inhibitor for improved cancer antiangiogenic therapy. The objectives of this Application are (i) to identify Ang- binding peptides that can selectively bind to and interfere with the regular functions of Ang by major coat phage display technique; and (ii) to construct a dual-functional recombinant phage nanofiber by double-displaying our recently discovered breast tumor-homing peptide at the tip and the identified Ang-binding peptide on the side- walls of the same nanofiber. The resultant phage nanofiber displaying both tumor-homing and Ang-binding peptides on its surface is expected to first home to breast tumors and then bind to the Ang within the tumors to inhibit the tumor angiogenesis. Our hypotheses are that (1) Ang-binding peptides can be identified from a major coat-displayed phage library, and (2) the dual-functional tumor-homing/Ang-binding phage nanofibers double- displaying Ang-binding and tumor-homing peptides can serve as a smart angiogenic inhibitor to selectively home to and retain within breast tumors, and bind Ang to block the tumor angiogenesis and subsequently inhibit the tumor growth. We will carry out two aims to test our hypothesis: Aim 1: Identify Ang-binding peptides from a major coat-displayed phage library by biopanning against Ang. We will use phage display technique to conduct biopanning against commercially purchased Ang and select Ang-binding peptides that can specifically target and bind to Ang from the major coat-displayed phage library. The Ang-binding specificity of the selected peptides will be verified by phage capture ELISA, peptide inhibition assay, and phage-Ang interaction assay. Aim 2: Construct dual functional breast tumor-homing/Ang-binding phage and evaluate its use in selectively inhibiting tumor growth. We will use our established double display technique to display both the breast tumor-homing and Ang- binding peptides on the single phage nanofiber and evaluate its antitumor activity in vitro. We will then inject the phage into the breast tumor models to verify that the phage can home to tumor, bind to Ang secreted by tumor cells to suppress angiogenesis and inhibit tumor growth. This project will lead to tumor-homing Ang-binding nanofibers that can serve as nanomedicines for selectively inhibiting cancer angiogenesis and progression without causing the side effects.